The present invention relates to a DPCM (differential pulse code modulation) system. DPCM systems are useful in applications such as video teleconferencing via satellite.
There is interest in using satellites to provide reliable video teleconferencing services using digital data transmission. However, due to power and bandwidth limitations of the satellite, it is important that the data rate be minimized consistent with acceptable picture quality. One way is to use DPCM wherein data is transmitted only when changes occur in the picture, e.g., from a pixel in a frame to a corresponding pixel in the next frame or one pixel in a line to a succeeding one in the same line. However, while it is especially important when using a satellite transmission channel, it is always desired to reduce the data rate to as low a value as possible regardless of the transmission channel used, i.e., satellite or terrestrial.
Even with typically low data rates produced by a DPCM system, a transmitter output buffer can be overloaded by the input data, e.g., when there is a large amount of motion in the picture. The buffer can provide a signal indicating that it is at or near capacity and this signal can be used to reduce the input data rate by reducing the number of quantizing levels and/or the number of samples. However, such a system degrades the picture resolution not only in picture areas having motion, where the degradation is acceptable since the resolution of the human eye degrades with increased motion, but also degrades resolution in stationary picture areas, where it may not be acceptable.
It is known from U.S. patent application Ser. No. 040,470, filed Apr. 17, 1987, and under common assignment herewith, to have a DPCM system with a fixed threshold.